There is known from EP 0 642 801 A1 a filtration device, which includes a collecting bag and a flexible-tube line system with a filter. The bag, which contains the fluid to be filtered, is not a component of the device. The flexible-tube line system of the known filtering device has, apart from the flexible-tube line into which the filter is connected, also a bypass line with a non-return valve, which branches off from the flexible-tube line upstream and downstream of the filter, so that after the end of the filtration process air can flow out of the collecting bag into the bag containing the fluid to be filtered. The exit-side chamber of the filter has an additional aeration opening, which is connected via a flexible-tube line with a drip chamber, which is connected into the flexible-tube line section leading to the filter.
A filtering system for biological or medical fluids is also known from WO 91/17809. The fluid to be filtered is transferred under the influence of gravity from a receiving bag via a flexible-tube line and a filter into a collecting bag. The drawback is that fluid still remains in the flexible-tube line and the filter after the end of the filtration process, i.e. after the fluid has ceased to flow into the collecting bag. In order to recover this fluid, there is provided on the flexible-tube line a gas inlet upstream of the filter and a gas outlet downstream of the filter. After the end of the filtration process, gas is conveyed into the system via the inlet, said gas displacing the liquid in the system as it is conveyed away again via the outlet.
In a preferred form of embodiment of the filtering system known from WO 91/17809, it is proposed, for the purpose of completely emptying the system, to provide on the flexible-tube line an outlet line upstream of the filter and a de-aeration line downstream of the filter. The aeration and de-aeration line lead into a common aeration and de-aeration bag, whereby hydrophobic membranes on both lines prevent fluid from flowing into the container. The filtering system is based on the principle that air is fed from the container via the outlet line to the system for the purpose of aerating the system and air is fed from the system via the de-aeration line to the container for the purpose of de-aerating the system. The connection points are therefore also designated as gas inlet and outlet.
EP 0 888 789 A1 describes a sterile system for the filtering of blood and blood components, which also makes use of a collapsible container, in which the air in the system is collected before the filtration process is started and used for the complete emptying of the system after the end of the filtration process. The filter of the known filtering system connected into the collecting line has two chambers, whereof the first chamber is connected to the collapsible container via a first flexible-tube line and the second chamber is connected to the collapsible container via a second flexible-tube line. Both flexible-tube lines are closed with a hydrophobic membrane. The fluid to be filtered flows into the collecting container thereby displacing the air in the two filter chambers through both flexible-tube lines into the collapsible container. For complete emptying of the system, the second flexible-tube line is first closed, whereby air flows from the collapsible container via the first flexible-tube line into the first chamber of the filter. The second flexible-tube line is then opened again, so that air flows via the second flexible-tube line into the second chamber of the filter. The effect of this is that the first filter chamber is aerated first and then the second filter chamber.
The known filtering system has been tried and tested in practice. The use of two sterile filters, which increase production costs, has however proved to be a drawback.